drm/i915: debugfs entries for [e]LLC

To make users life a little easier figuring out what they have on their
system.

Ideally, I'd really like to report LLC size, but it turned out to be a
bit of a pain. Maybe I'll revisit it in the future.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>